There are occasions on which such material is dissolved in different solvents, (or in mixtures of two or more solvents in different proportions), and is to be recovered by simultaneous evaporation of the solvents. One example is the output from a preparative HPLC machine. Where material has been dissolved in a composite solvent having two or more components which vary in proportion from one sample to another (e.g. water and acetonitrile in varying proportions), the solvent can vary in the example given from almost pure water to almost pure acetonitrile. Acetonitrile is more volatile than water and has a lower heat of vaporisation so that it evaporates faster in a centrifugal evaporator leading to differential sample masses during evaporation. This leads to two problems.
Firstly out of balance loads can be generated. Various techniques have been proposed to deal with this first problem.
Secondly there is the possibility of overheating samples which dry before others. This can arise where material is dissolved in solvent mixtures containing a higher proportion of the more volatile component than in other samples. In this situation continued heating of the samples to complete the evaporation of the solvent mixtures containing a higher proportion of the less volatile component can result in the dried material becoming overheated. Samples that are still evaporating will not rise above the temperature of the evaporating liquid, but any dry samples can reach temperatures which are high enough to cause damage to the residue material if precautions are not taken to protect that material.
In one arrangement designed to try and overcome this problem, it has been proposed to mount sample containers in four or more aluminium blocks in swinging buckets in an evaporator, as illustrated in FIG. 1. Here the aluminium block is denoted by reference numeral 10, and one of the sample containers by 12. The block is located in a bucket 14 which is free to swing about a pivot 16 so that when the rotor 18 is stationary the buckets hang vertically as shown on the left hand side of FIG. 1. When the rotor 18 spins up to speed by rotating the vertical shaft 20, the buckets swing upwardly and outwardly from the position shown on the left hand side of FIG. 1, and take up a horizontal position under the influence of centrifugal force, as shown on the right hand side of FIG. 1.
Overheating is prevented by sensing the temperature of one of the blocks and controlling the heat input to the rotating assembly so as to maintain the temperature at or below the maximum permissible dry material temperature. Provided the distribution of differing solvent mixtures throughout all the samples in all the blocks is uniform (which unfortunately cannot be guaranteed), this may protect any samples that become dry. But in cases in which the material is dissolved in solvent mixtures of randomly variable composition (as is more often the case), a block which is not temperature controlled, may contain relatively more samples containing a high proportion of the more volatile component, and all the samples in it may become dry, while samples in the block whose temperature is being controlled may still contain solvent. In those circumstances the block whose temperature is not being controlled, may rise in temperature to a value at which damage can occur to the dry residue sample material therein.
This problem could be overcome by incorporating a temperature sensor in every sample block, and controlling the heat to each block independently so as to keep each block below the maximum permissible temperature. However this can be expensive and difficult to implement especially if there are a large number of sample blocks.